1. Field of the Invention
The present invention relates to a process for forming a semiconductor device on a transparent dielectric substrate such as a sapphire substrate, more particularly to the formation of a reflective film on the transparent dielectric substrate to enable the substrate to be recognized optically.
2. Description of the Related Art
Semiconductor integrated circuits formed in silicon films grown on sapphire substrates are advantageous for applications in environments in which radiation poses a hazard. Such silicon-on-sapphire (SOS) integrated circuits are generally formed by use of conventional fabrication equipment of the type that creates semiconductor integrated circuits in semiconductor substrates. In conventional fabrication processes, the fabrication equipment often uses optical sensors to detect the position of the semiconductor substrate. The position of a sapphire substrate cannot be detected in this way because sapphire is transparent: light passes straight through the substrate instead of being reflected back to the sensor. One known solution to this problem is to coat the sapphire substrate with a light-reflecting film.
Japanese Patent Application Publication No. 7-283383 and the parent U.S. Pat. No. 5,877,094, for example, describe a sapphire substrate coated on its backside with a layer of polycrystalline silicon (polysilicon) at least about two micrometers (2 μm) thick, which reflects light and can be detected optically. Phosphorous ions are also implanted into selected regions of the polysilicon film to form conductive doped regions that can be detected electrically.
One problem with this substrate is that forming a polysilicon layer at least about 2 μm thick is a time-consuming and therefore expensive process. Moreover, in reflow and other subsequent heating steps, the large difference in thermal expansion coefficients between sapphire and polysilicon may cause the sapphire substrate to warp. Such warping interferes with the fabrication process and may lead to the formation of cracks in the sapphire substrate, particularly if the sapphire substrate is thin, which is the current trend.
Japanese Patent Application Publication No. 11-220114 describes an SOS substrate having an optically reflecting polysilicon coating 0.5 μm to 3.0 μm thick on its backside. A pattern of cuts is formed in the reflective coating so that the difference in thermal expansion coefficients does not cause the substrate to warp or crack. The thickness of the polysilicon coating must be at least 0.5 μm because a thinner film would lack the necessary reflectivity, as pointed out in paragraph 0009 of the above disclosure.
Due to the trend toward thinner sapphire substrates, there is a continuing need for still thinner reflective films.